Carrier aggregation is a feature recently developed by the members of the 3rd-Generation Partnership Project (3GPP) for so-called Long Term Evolution (LTE) systems, and is standardized as part of LTE Release 10, which is also known as LTE-Advanced. An earlier version of the LTE standards, LTE Release 8, supports radio link bandwidths up to 20 MHz. In LTE-Advanced, bandwidths up to 100 MHz are supported. The very high data rates contemplated for LTE-Advanced will require this expansion of the transmission bandwidth.
In order to maintain backward compatibility with LTE Release 8 user terminals, the spectrum available for use by systems and mobile terminals compatible with the LTE-Advanced standards is divided into Release 8-compatible chunks called component carriers. LTE-Advanced mobile terminals (referred to as “user equipment” or “UEs” in 3GPP documentation) and systems can “aggregate” two or more of these component carriers to achieve higher bandwidth and higher data-rate transmissions than are possible in an LTE Release 8 system.
This approach, called “carrier aggregation,” thus enables bandwidth expansion beyond the limits of LTE Release 8 systems by allowing user terminals to transmit data over multiple component carriers that together can cover up to 100 MHz of spectrum. This is shown in FIG. 1, which illustrates the aggregation of five 20-MHz bands Importantly, the carrier aggregation approach ensures compatibility with earlier Release 8 terminals, each of which may simultaneously use one of the five 20-MHz bands, while also ensuring efficient use of a wide carrier by making it possible for legacy terminals to be scheduled in all parts of the wideband LTE-Advanced carrier.
Release 10 of the LTE standards supports up to five aggregated carriers, where each carrier is limited to one of six radio frequency (RF) bandwidths, corresponding to 6, 15, 25, 50, 75 or 100 LTE resource blocks. The respective RF bandwidths are 1.4, 3, 5, 10, 15, and 20 MHz. Carrier aggregation is called intra band if the carriers all belong to the same 3GPP operating band. The carriers in the same band may be contiguous, as shown in FIG. 2, or non-contiguous, as shown in FIG. 3. Inter-band carrier aggregation is the case when there is at least one carrier in a different 3GPP operating band. This is shown in FIG. 4.
The number of aggregated component carriers, as well as the bandwidth of the individual component carrier, may be different for uplink (UL) and downlink (DL) transmissions. A symmetric configuration refers to the case where the number of component carriers in downlink and uplink is the same. An asymmetric configuration refers to the case where the number of component carriers is different. However, an asymmetric configuration where the number of uplink component carriers is higher than the number of downlink component carriers is not allowed under the Release 10 specifications.
The number of component carriers configured for a geographic cell area may be different from the number of component carriers actually “seen” (or used) by a given terminal. A user terminal, for example, may support more downlink component carriers than uplink component carriers, even though the same number of uplink and downlink component carriers may be actually offered by the network in a particular area.
From a network deployment perspective, during the timeframe in which Release 10 equipment is deployed, all cells are expected to be compatible with Releases 8 and 9 of the LTE specifications. During initial access, a LTE Release 10 terminal behaves in the same way as a LTE Release 8/9 terminal, and accesses a single cell using a single uplink carrier and single downlink carrier. The serving cell in which the UE end up at initial access is referred to as the UE's Primary Cell (PCell). After successful connection to the network a terminal may—depending on its own capabilities and the network—be configured with additional component carriers in the UL and DL. Additional serving cells that are configured for the UE are referred to as Secondary Cells (SCell).
Configuration of SCells is performed using Radio Resource Control (RRC) signaling. Due to the heavy signaling and rather slow speed of RRC signaling, it is not desirable to frequently change the configured cells for a given mobile terminal. Accordingly, the specifications are designed to allow a terminal to be configured with multiple component carriers even though not all of them are currently used. Secondary cells may be activated and deactivated, using Medium Access Control (MAC) signaling, as needed. Thus, to make a downlink transmission on an configured SCell or to be able to provide the SCell with an uplink grant the serving base station (an evolved Node B or “eNB” in 3gPP terminology) must first activate the SCell by sending an activation/deactivation command (as a MAC CE) to the UE.
MAC signaling is much faster than RRC signaling, allowing rapid activation and deactivation of component carriers. When a terminal is activated on multiple SCells, this implies that it has to monitor all DL component carriers for PDCCH and PDSCH. This implies a wider receiver bandwidth, higher sampling rates, etc., resulting in high power consumption. While a component carrier is deactivated, related circuitry and/or procedures may be disabled, allowing the mobile terminal to reduce its power consumption.
Differences Between Primary and Secondary Cells when Using CA
As suggested above, the Primary cell (PCell) for an LTE-Advanced mobile terminal (UE) basically corresponds to the Release 8/9 “serving cell.” The UE monitors system information only on the PCell, and also takes security and Network Access Stratum (NAS) mobility information from this cell. System information needed by the UE to properly use a SCell is provided to the UE via dedicated signaling.
An uplink SCell does not have any Physical Uplink Control Channel (PUCCH) of its own, but the UE may transmit hybrid-ARQ (HARQ) acknowledgements/negative-acknowledgements (A/N) and Channel State Information (CSI) for SCells on the PUCCH of the PCell. Cross-carrier scheduling, whereby the UE is allocated resources for an SCell via a Physical Downlink Control Channel (PDCCH) of another serving cell used by the same UE, is not supported for the PCell. Semi-Persistent Scheduling (SPS) and Transmission Time Interval (TTI) bundling is supported only on the PCell. Likewise, the Radio Link Monitoring and Radio Link Failure functionalities specified for LTE apply only to the PCell.
Furthermore, the PCell for a given mobile terminal always consists of a DL carrier and an UL carrier linked to the primary DL carrier via a System Information Block Type 2 (SIB2), while an SCell may be consist of a DL carrier and a corresponding SIB2-linked UL carrier, or only a DL carrier. From eNB perspective, at least in the Release 10 time frame, all serving cells will still consist of a DL carrier and the SIB2-linked UL, even if some mobile terminals served by the eNB are configured with an SCell that consists of only a DL carrier.
The PCell for a given mobile terminal can be changed using handover (HO) procedures. During HO, all SCells configured for the mobile terminal being handed over are deactivated. The target eNB (which may be the same as the source eNB) then decides whether to use the same SCells, configure and activate a different set, or to simply deconfigure them.
From the mobile terminal perspective, the PCell is always activated, while SCells can be activated and deactivated on a need basis. Functionalities with respect to activated and deactivated SCells is described further in chapter 5.13 of the 3GPP document “Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification,” 3GPP TS 36.321, v. 10.5.0 (March 2012), available at www.3gpp.org. As noted above, activation and deactivation of SCells is performed using a MAC Control Element (MAC CE), while configuration and deconfiguration of SCells is performed using RRC signaling.
A mobile terminal capable of carrier aggregation can have only one PCell and up to four SCells. The Cell Radio Network Terminal Identifier (C-RNTI) used by the mobile terminal (e.g., to scramble and de-scramble data at the physical layer) is UE-specific and the same C-RNTI is used both in the PCell and in the SCell(s).
UE Capability Signaling
Mobile terminals compliant with Releases 8 and 9 of the LTE standard are able to signal which bands they support, using a UE Capability Information Element (IE) specified in “Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol Specification,” 3GPP TS 36.331, v. 9.10.0 (March 2012), available at www.3gpp.org. In the Release 10 version of the specifications (e.g., 3GPP TS 36.331, v. 10.5.0 (March 2012)), the content of this IE was extended when Carrier Aggregation was introduced, in order to allow the UE to indicate support for combinations of bands (i.e, to be able to indicate support for aggregation of serving cells from different or same bands). Note that a Release 10 UE is not required to support carrier aggregation, as it is an optional feature.
As part of the signaled band-combination the UE can also indicate, for a specific band combination, whether it supports only a DL Scell in a specific band. Support of UL-only SCells is not supported in the Release 10 timeframe. As part of the signaled band-combination, the UE also indicates which bandwidth class applies and whether MIMO is supported. The following is excerpted from 3GPP TS 36.331, v. 10.5.0 (March 2012):
BandParameters-r10 ::= SEQUENCE {bandEUTRA-r10INTEGER (1..64),bandParametersUL-r10BandParametersUL-r10OPTIONAL,bandParametersDL-r10BandParametersDL-r10OPTIONAL}
For example, for a UE which has RF transmitters capable of supporting both band 4 and band 17, but only supports CA when band 4 is the PCell, the configuration might look like this:
SupportedBandCombination-r10 {BandCombinationParameters-r10_1,BandCombinationParameters-r10_2,BandCombinationParameters-r10_3}WhereBandCombinationParameters-r10_1 {BandParameters-r10_1_1,BandParameters-r10_ 1_2}BandCombinationParameters-r10_2 {BandParameters-r10_2_1}BandCombinationParameters-r10_3 {BandParameters-r10_3_1}WhereBandParameters-r10_1_1 {bandEUTRA-r10 = 4bandParametersUL-r10 = BandParametersUL-r10bandParametersDL-r10 = BandParametersDL-r10}BandParameters-r10_1_2 {bandEUTRA-r10 = 17bandParametersUL-r10 = NOT PRESENTbandParametersDL-r10 = BandParametersDL-r10}−> UE states that it supports having the PCell in B4 and the SCell withDL only in B17BandParameters-r10_2_1 {bandEUTRA-r10 = 4bandParametersUL-r10 = BandParametersUL-r10bandParametersDL-r10 = BandParametersDL-r10}−> UE states that it supports having the PCell in B4 and no SCellconfiguredBandParameters-r10_3_1 {bandEUTRA-r10 = 17bandParametersUL-r10 = BandParametersUL-r10bandParametersDL-r10 = BandParametersDL-r10}−> UE states that it supports having the PCell in B17 and no SCellconfigured
Thus, with the current 3GPP standards, it is possible for the UE to signal that it supports a particular band combination. However, improved solutions to allow the best use of all possible configurations are needed.